You might check with Smith and Wesson about ordering a replacement cylinder assembly, and a full crane. I doubt they would install 'just' a cylinder for you. It would be a mis-match to the indicated caliber on the barrel shroud, and they'd probably tell you that accuracy would suffer (not that its' a target-gun, LOL). With a full assembly, you'd just have to remove the front retaining screw each time you wanted to swap the calibers.
